1. Field of the Invention
This invention relates to a process of preparing a solid granular diammonium phosphate, and more particularly to a process including a diversion step after the drying step and prior to the classifying step to enhance the production of a particular range of granular sizes of solid diammonium phosphate within the broad production range.
2. Information Disclosure Statement
Granular diammonium phosphate is conventionally prepared by reacting an aqueous solution of phosphoric acid with ammonia to prepare a partially reacted slurry of monoammonium phosphate and diammonium phosphate (also referred to as "DAP"). The slurry is pumped to a rotary granulator-reactor where further ammonia is added to yield a diammonium phosphate product. The diammonium phosphate product leaving the grandulator-reactor is transferred to a dryer. After drying, the granular product is separated by a classifying means, such as, a set of screens including an oversize screen and a product screen to separate out the oversize and undersize granular diammonium phosphate from the desired granular diammonium phosphate product. The recycle may include a portion of product received from the classifying screens. In the conventional process the size of the granule produced depends largely upon the granulator. Generally, the granulator produces a relatively wide or broad range of granular sizes and which, for the most part, does not vary within the range. This range is peculiar to a particular granulator although, increasing the recycle tends to decrease the size of the granule produced and to decrease the ability to make a narrow product cut from the entire process output.
According to at least one prior art process, it is desirable to decrease the amount of recycle since the amount of recycle interferes with plant capacity. High recycle rates are also discouraged because of the high investment in equipment for the purpose of dealing with the recycle.
To minimize the amount of recycle a prior art process utilizes a kneading mill to prevent agglomeration of the larger particles thereby preventing formation of even larger sized particles.
U.S. Pat. No. 3,199,973 teaches a process of preparing diammonium phosphate utilizing a recycle step which includes the milled oversizes and fines from the screening process recycled to the dryer.
U.S. Pat. No. 2,946,655 teaches a process for the preparation of diammonium phosphate where the classifying step separates the product from the coarse and fines and returns the milled coarse and fines back to the heater to supply the solid diammonium phosphate particle for bringing the solution to the saturation state.
U.S. Pat. No. 3,249,421 teaches the importance of reducing the amount of recycle and teaches a process for preparing diammonium phosphate which recycles the fine, mills the coarses and recycles the milled coarses with the fines to the granulator.
U.S. Pat. No. 3,153,574 teaches a process for the production of granular diammonium phosphate which utilizes a recycle step recycling the fines obtained from the screening process to the ammoniator-granulator. It is noted that the fines are of a predetermined size.
U.S. Pat. No. 2,999,006 teaches a method for preparing substantially dry diammonium phosphate from monoammonium phosphate and gaseous anhydrous ammonia.
U.S. Pat. No. 3,249,421 teaches a method of preparing a granulated fertilizer containing substantial or a large amount of diammonium phosphate. The process includes a recycle step which recycles the milled oversize and the fines obtained from the classifying means back to the granulator. A substantial portion of the medium size particles obtained from the classifying means are transported to storage. Any remaining medium size particles obtained from the classifying means may be recycled through the granulator.
U.S. Pat. No. 3,485,580 teaches a method for producing granular diammonium phosphate. The process recycles the milled oversize and the fines obtained from the separating means back to the blunger.
U.S. Pat. No. 2,963,359 teaches a process for preparing diammonium phosphate which recycles the fines and grinds the oversize and part of the product in a grinder and recycles the fines and ground oversize end product to the granulator. The purpose of the diammonium phosphate being added to the granulator is to provide a bed for the carrier of phosphoric acid and is in an amount of about 3 times the product made. The particles of the phosphate making up the beds are properly sized or ground to a suitable fineness to finish sufficient carrier and to supply surface area for the liquid phosphoric acid and the furnish proper nuclei for granulation.
U.S. Pat. No. 3,514,255 teaches the production of granular diammonium phosphate where the recycle step uses the diammonium phosphate fines obtained from the screening process and from the milled oversize which are reacted with the phosphoric acid feed stream to produce an intermediate solid granular stream which is substantially ammoniated by reaction with the ammonia feed in a separate step.
Defensive publication No. T101,203 published Nov. 3, 1981 teaches a process for the production of granular diammonium phosphate which recycles the fines obtained from the screening process to the drum granulator.
It is an object of this invention to produce and collect a large percentage of a relatively narrow range of DAP particle sizes without increasing the percentage of undersize or oversize and without overloading the classifying system.
It is a further object of this invention to provide a process for the manufacture of solid granular diammonium phosphate which results in a homogeneous, closely sized, granular product with a high ratio of nitrogen to P.sub.2 O.sub.2.
It is a further object of this invention to provide an improved method for preparing solid granular diammonium phosphate without broading the range of granular sizes produced while maintaining the desired narrow product granule range and not overloading the screening system.
It is a further object of this invention to provide a method of preparing a solid granular diammonium phosphate with a relatively uniform particle size.
It is a further object of this invention to increase the production by approximately 30% of 2-4 mm granular diammonium phosphate over the prior art process.
It is a further object of this invention to shift within the broad range of DAP particle size produced by the conventional process by enhancing the production of a narrow range of DAP particle size distribution by manipulating the percent of DAP product mixture diverted directly from the dryer and introduced to the granulator-reactor while maintaining a narrow cut of DAP product collected by the classifying means.
It is a further object of this invention to prepare a narrow separation of diammonium phosphate granule which stores better and has less breakage than the diammonium phosphate granule obtained from the process without a diversion step.
It is a further object of this invention to increase the effective screening capability of the separation system and reduce the milling requirements of the recycle system.
It is a further object of this invention to provide an increase in the production of 2-4 mm granular diammonium phosphate by diverting, without classifying, approximately 30 to 50 percent of the diammonium phosphate product mixture prior to delivery to a screening means.
It is a further object of this invention to provide an increased granular particle size without increasing the percentage of oversize or undersize.
It is a further object of this invention to provide a change in the materials handling system to result in a more uniform product particle size of solid diammonium phosphate.
It is an object of this invention to increase the productivity of the desired 2-4 mm granular diammonium phosphate product over the production of 2-4 mm diammonium phosphate without diversion of dried diammonium phosphate prior to introducing the unclassified diammonium phosphate to the classifying means.
It is an advantage of this invention to reduce the cost in obtaining the desired granular size by increasing the percentage of desired product produced by the process of preparing diammonium phosphate.
It is an advantage of this invention to reduce the amount of oversize to be milled thereby decreasing electrical and maintenance costs in operating the plant.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more pertinent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention is a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawing.